1. Field of the Invention
The present invention relates to a web tensioning control system, and more particularly to a control system for controlling a web tensioning apparatus in which a web of material, such as a textile web or the like, is pulled through a vat of liquid by an exit motor driving a corresponding pinch or nip roller. In order to compensate for various drag components on the web, a plurality of helper motors are provided at regular intervals within the web to push the web along. The inventive control system and component circuits are designed to provide uniform tension throughout the web by reliably and efficiently controlling the tension applied to the web by each helper motor based upon a tension feedback readout from a single tension sensor located near the exit motor.
2. Description of the Related Art
Web tensioning systems, particularly systems in which a web of material is pulled through a vat of liquid by a exit motor driving a pinch or nip roller, are well known for the treatment of textiles and the like. In many such systems, in order to attempt to provide a uniform tension on the web throughout the vat, a plurality of helper motors are arrayed above the vat at regular intervals to compensate for various drag components on the web. A plurality of guide rollers are immersed in the vat, with the material web alternately partially encircling one or more guide rollers and thence through a helper motor pinch roller such that the continuous web follows a complex path through the vat to maximize exposure of the material to the liquid.
It is known to provide individual controls for each helper motor to allow the helper motors to make differing load compensation contributions depending upon sensed web tension values at the helper motor locations. In general, such individual controls require separate tension sensors at each or most helper motor locations, and also require each helper motor to be sized to compensate for any drag induced throughout the entire system. Thus, a number of relatively large horsepower helper motors and a plurality of corresponding tensiometers must be used to generate helper motor control signals. Furthermore, in known systems, the helper motors are only controllable via a web tension feedback and thus the helper motors cannot be operated via a speed based control during threading and emergency conditions.
An example of such a prior art system is disclosed in U.S. Pat. No. 4,645,109 to Fleissner. Systems such as Fleissner's require an unacceptably high initial equipment expense for the large motors and corresponding tensiometers and continuing large maintenance costs for maintaining such an array of tensiometers, which are prone to breakage and readout error.
Furthermore, upon initial start-up of a system such as Fleissner's, drag components on the web are constantly shifting due to acceleration forces, and analog motor control circuits tend to overcompensate for the sensed tension errors, often resulting in wide tension fluctuations. This can result in tearing of breaking the web, which means that the entire system must be shut down and drained so that the web can be laboriously rethreaded through the pinch rollers and guide rollers. This leads to another problem inherent in prior art systems, i.e. in the event of web breakage, a control system based solely upon tension sensing can overspeed as a result of web breakage or during initial material feed, when there is no tension to be sensed. This can result in the motors literally throwing the material web out of the vat at high speed with consequent danger to surrounding personnel and machinery.
It is clear then, that a need exists for a precise, digitally based web tensioning control system in which a plurality of helper motors can be sized to compensate for induced material drag only within their corresponding sector of the web. Such a control system should allow control parameters to be instantaneously switched between a web tension based feedback control and a web speed based feedback control for those situations in which no web tension is present. Such a control system should also be capable of "soft starts" whereby widely varying tension conditions due to acceleration during start-up are not chased wildly, with consequent damage to or breakage of the material web. Finally, such a control system should provide individual helper motor tension control signals based solely upon a single exit tension sensor readout, which motor control signals result in a relatively uniform tension throughout the material web.